1. Field of the Invention
This invention relates basically to a method and apparatus for deforming a metal workpiece by passing a stress wave through the workpiece sufficent to render the metal momentarily plastic. More specifically, this invention relates to a method and apparatus for converting electromagnetic energy into a stress wave which is amplified, focused and directed to a metal workpiece such that the metal is in a plastic state when the stress wave passes through the workpiece.
2. Description of the Prior Art
At the present time, the various methods of metal forming basically employ energy in the form of an externally applied force or heat. An illustration of this can be found in metal forging, where an external force or impact is applied to a metal workpiece, which may be hot or cold, in order to form it into the desired shape. Another illustration is that where metal is heated to melting and then poured as a fluid into a mold and allowed to cool.
In the case of metal forging, a great amount of energy is required to do the work which in turn necessitates the utilization of large and bulky machinery. This is true whether the metal is cold-worked or hot-worked although less force is required in hot-working. Examples of such machinery include power presses, hydraulic presses, drop hammers, and steam hammers. Another metal working process employing similar techniques is riveting. Pneumatic hammers and stationary presses are used in making riveted joints, the press giving the structurally better joint.
The distinction between cold-working and hot-working of metal rests on the relationship of the processing temperature to the recrystallization temperature of the metal. The recrystallization temperature is that temperature at which there is a marked softening of the metal being worked. Cold-working of a metal is the deformation of the metal at a temperature below the recrystallization point. More power is required for cold-deformation since the metal is harder and less ductile than during hot-working.
As noted above, the cold-working technique has been and is used to upset rivets. These methods produce results which are not wholly successful. Rivet failures are not uncommon and occur unpredictably in operation. Tests done with various conventional riveting machines, which upset rivets by cold-working, indicate wide inconsistencies of failure under load. This illustrates only one instance where the present cold-working methods are inadequate and require improvement.
Another method of metal forming is by use of an expanding magnetic field as illustrated by the patent to Harvey et al, Ser. No. 2,976,907, issued Mar. 28, 1961. The Harvey patent discloses the use of a magnetic field established by a coil to create a pressure or force on the metal workpiece thereby deforming it to the desired shape.
More recently, laboratory investigation and study has indicated that metals may be deformed when subjected to stress waves. For example, see "Large Deformation Dynamic Plasticity At An Elastic-Plastic Interface," by J. F. Bell appearing in J. Mech. Phys. Solids, 1968, Vol. 16, p. 295. In this report it is shown that a state of plasticity can be established in metal by a stress wave generated by striking two hardened metal bars together.
It should also be noted that the present state of the art includes an apparatus which utilizes the electromagnetic. repulsion of two magnetic fields and the shock produced therefrom to hold a metal workpiece in position while it is deformed by other means. (Harvey et al, U.S. Pat. No. 3,108,325, FIG. 1, issued Oct. 29, 1963.) This technique, however, does not utilize a stress wave which renders the metal plastic to accomplish the deformation.
It has not heretofore been possible to form metallic objects by the utilization of a stress wave passing through the metal except as an object of scientific study. There are no commercial methods which utilize this concept nor are there any commercial or scientific methods which utilize electromagnetic forces to generate a stress wave sufficient to render metal momentarily plastic.